Various methods are known for imparting catalytic activity to carbonaceous chars by treatment with nitrogen-containing compounds. In some treatments a high-temperature char such as a charcoal or activated carbon is heated at temperatures above 700.degree. C. in the presence of a nitrogen-containing compound such as ammonia or an amine. In other treatments activation of the char with steam and exposure to the nitrogen-containing compound occur simultaneously. Other processes incorporate the nitrogen-containing compound directly into the raw material used to produce the char. In an especially effective method, the nitrogen-containing compound is introduced after low-temperature carbonization and oxidation of a nitrogen-poor char feedstock but before high temperature exposure and condensation of the carbon structure. This method produces the highest known functional utility with the least economic and environmental costs. For the purposes herein all carbons produced by such processes will be referred to as "nitrogen-treated carbons."
The nitrogen-treated carbons have the ability to function as catalysts per se without the addition of metal impregnants. They have utility in a number of applications such as sulfide oxidation and peroxide decomposition. During use the nitrogen-treated carbon can become deactivated, losing their catalytic activity and requiring that they be replaced with fresh catalyst material.
The mechanism of deactivation is not known or understood. However, it has been postulated that two principal means of deactivation may be involved. These postulates are based upon analogies to other systems employing heterogeneous catalysis. In one case, the carbon catalyst is thought to become deactivated as a result of occlusion of the catalyst sites by material physically adsorbed on or in the carbon. These materials may be reaction products or secondary adsorbates unrelated to the catalysis reactions. This mechanism is hereinafter referred to as "site occlusion."
The other postulated case suggests that the catalyst sites become deactivated by direct reaction and poisoning with moieties involved in the catalysis or with secondary materials which act only as site poisons. This mechanism is hereinafter referred to as "site poisoning." Most cases of catalyst deactivation can be accounted for by some combination of the site occlusion and site poisoning mechanisms. Of these two mechanisms, site poisoning is believed to be the most serious since it involves a fundamental change in the reactive properties of the catalyst site.
Very few methods are known for the restoration of catalytic activity in deactivated nitrogen-treated carbon catalysts. Those methods involve primarily low-temperature thermal treatments, i.e. treatments conducted at temperatures below about 700.degree. C., and more particularly below 500.degree. C. The main source of catalyst deactivation in such cases has been the presence of sulfur oxide compounds (SO.sub.x) which can be readily removed in that temperature range. Other interferents, such as organic hydrocarbons, are largely absent. The primary mechanism of catalyst deactivation in these instances appears to be one of site occlusion.
One example of reactivation above 700.degree. C. involves the removal of oxalic acid or water to restore catalytic activity. Again, the primary mechanism of deactivation appears to be one of site occlusion. However, since oxalic acid decomposes at only about 189.degree. C. into formic acid (b.p. 101 C.) and carbon dioxide, and since water vaporizes at 100.degree. C., this does not appear to be representative of a reactivation process which requires temperatures above about 700.degree. C. Therefore, the general utility of these known methods in this area is unknown. This is thought to be because high temperature treatments have certain features which would be expected to diminish the recovered activity of the reactivated catalyst. High temperature treatments in steam or other oxidizing agents, for example, would be expected to cause losses in carbon mass and, therefore, significant losses in recovered catalytic activity. Also, since the catalytic sites appear to function primarily as sites for oxidative catalysis, there is a possibility that such sites will preferentially catalyze their own destruction at high temperatures. On the other hand, high temperature treatments under inert conditions can lead to extensive cracking of organic hydrocarbon adsorbates. Organic materials such as these can be common constituents in many process streams. Such cracking would lead to the deposition of pyrolytic carbon on the catalyst sites. The result of this deposition could also be deactivation of the catalyst sites. This type of deactivation is common in many conventional catalyst applications.
Notwithstanding the problems involved, high-temperature treatment is generally desirable in those cases where a significant proportion of site deactivation occurs as a result of occludates or poisons that can be removed from the carbon surface in no other convenient manner. For example, it has been observed that oxygen can poison the catalyst sites in nitrogen-treated carbon catalysts at temperatures above ambient. The degree of poisoning increases with increasing oxygen exposure and increasing exposure temperatures. Such poisoning becomes particularly extensive at temperatures above about 400.degree. C. Therefore, low temperature thermal treatment as practiced by known art would be unable to restore significant catalytic activity in such cases and may, in fact, accentuate the problem.
Accordingly, it is the object of the present invention to provide a method for reactivating spent nitrogen-treated carbonaceous chars by the use of high temperature thermal treatments which can remove both catalyst site poisons and occludates and restore activity to the material. It is a further object of the present invention to provide a method for reactivation which is compatible with process equipment and practices currently available for the high temperature treatment of non-nitrogen treated carbon materials. In this way reactivation costs can be minimized and the utility of conventional high temperature treatment equipment greatly extended.